This invention relates to a method for the removal of carbon compounds from the Bayer liquor in the manufacture of alumina by the Bayer Process.
The Bayer Process for the manufacture of alunina is a procedure which comprises dissolving an alumina-containing ore, usually bauxite, in conjunction with an aqueous caustic soda solution in a digester thereby causing the alumina component of the bauxite to be extracted in the form of sodium aluminate into the liquid, separating the insoluble component (called "red mud") from the liquid, adding a seed to the supernatant sodium aluminate remaining after the separation of the red mud, decomposing the sodium aluminate to precipitate alumina hydrate, isolating the alumina hydrate and calcining the isolated alumina hydrate to produce alumina. The hydrated alumina in its uncalcined form finds utility as the raw material for various aluminum compounds and for activated alumina and as the filler for synthetic resins.
The liquid which remains after the separation of the alumina hydrate is circulated for re-use in the Bayer Process.
As is evident from the explanation given above, in the system operated for the manufacture of alumina by the Bayer Process, the liquids containing the sodium component and the aluminum component are recycled. In the present invention, all these liquids are collectively referred to as "Bayer liquor". This invention aims to provide a method capable of removing the carbon compounds which come to be contained in the Bayer liquor for various causes to be described afterward. The term "carbon compounds" as used in the present invention refers to sodium carbonate, sodium oxalate and all other organic and inorganic compounds which contain carbon atoms.
Carbon compounds pass into the Bayer liquid roughly by the following major causes.
(1) Bauxite: Bauxite is a natural ore which contains various carbon compounds present in soil. Bauxite produced in tropical zones is dug out in open cut mines and since the ore is mined simply by stripping a shallow surface layer of the earth's crust, it contains vegetable components in a relatively large amount. Although the content and kind of carbon compounds contained in bauxite vary with the variety of bauxite, the bauxite generally contains 0.1 to 0.5% by weight, as carbon, of such carbon compounds. Specifically, they are carbonates of alkaline earth elements or, more frequently, organic compounds such as humin, bitumen and butin which have molecular weights varying over a wide range.
(2) Flocculant: A synthetic high-molecular flocculant such as a polyacrylate or polyacrylamide or a natural flocculant such as starch is used for the purpose of accelerating sedimentary separation during the separation of red mud or the separation of precipitated alumina hydrate.
(3) Defoaming agent: The Bayer liquor foams which it takes in an organic substance. To preclude this foaming, a defoaming agent which is a carbon compound such as a fatty acid ester is used at various stages throughout process.
(4) Carbon dioxide gas present in air: Air lift agitation is usually employed for facilitating the precipitation of alumina hydrate. In this case, the liquid being agitated absorbs carbon dioxide gas from air and a carbonate is produced.
(5) Caustic alkali as raw material: Generally about 100 kg equivalent of caustic soda is used for each ton of alumina to be produced. This amount of caustic soda contains 0.1 to 10 g/lit. of sodium carbonate.
A part of the carbon compounds which have passed into the Bayer liquid from the various sources enumerated above are discharged from the Bayer liquor through the following paths.
(1) A part of the carbon compounds contained in the bauxite escape dissolution and pass in their unaltered form into the red mud. The greater part of the flocculant used for accelerating the sedimentary separation of red mud is discharged from the liquid together with the red mud. A small amount of the Bayer liquor is deposited on the red mud and discharged together therewith out of the system. In the stage of bauxite digestion, a minor or major part of kaolinitic silicon oxide and quartz present in the bauxite, upon dissolution, react with caustic soda and re-precipitate themselves as sodalitic solids (desilication product) and eventually are discharged as part of the red mud. Carbon compounds in the form of carbonates, for example, find their way into these solids and are discharged together therewith.
(2) The alumina hydrate is separated by filtration and washing. The filtered alumina hydrate has about 10% of water adhering thereto. A small amount of carbon compounds are dissolved in the water so adhering to the alumina hydrate. Organic acids are part of the main carbon compounds present in the Bayer liquor. As the total concentration of such organic acids increases, sodium salts of such organic acids are isolated during the precipitation of alumina hydrate. These sodium salts, either in their unaltered form or after having formed nuclei for the crystallization of alumina hydrate, are discharged out of the system in conjunction with the alumina hydrate.
(3) Chiefly in the stage of bauxite digestion, part of the carbon compounds are gasified (preponderantly into carbon dioxide gas) through reaction with caustic soda and discharged out of the system.
(4) Part of carbon compounds are deposited in the form of scale on the inner wall surface of the production equipment and, thus, are discharged out of the system.
Many of the organic compounds present in the bauxite are dissolved in the caustic soda solution in the digestion stage held at elevated temperatures (130.degree. to 300.degree. C.). At first the dissolved organic compounds are high-molecular compounds. As they undergo successive decomposition under pressure and heat, they are gradually converted into low-molecular intermediate carbon compounds such as succinic acid and acetic acid. Eventually the major part of them are turned substantially into sodium oxalate and sodium carbonate. As a result, the carbon compounds present in the Bayer liquor consist of sodium oxalate, sodium carbonate, high-molecular organic compounds originally entrained by the liquid and intermediate carbon compounds. They accumulate in the Bayer liquid to an increasingly higher total concentration.
Besides, the high-molecular flocculant used for accelerating the sedimentary separation of the precipitated alumina hydrate is entrained by the finely divided alumina trihydrate crystals serving as the seed for precipitation of alumina hydrate and circulated throughout the production system. During the circulation through the system, the flocculant undergoes successive chemical changes and keeps on growing in amount in the system.
The total concentration of carbon compounds present in the Bayer liquid is determined by the amounts of various source substances entering into or departing from the liquid as described above.
Presence of carbon compounds in the Bayer liquor hinders the Bayer Process in the following ways:
(1) The rate of precipitation of alumina hydrate is lowered. Since the rate of precipitation is affected by various factors, the change in the actual amount of precipitation cannot be accurately estimated from only the amount of carbon compounds present. It has been demonstrated empirically, however, that the rate of precipitation is lowered by 0.2 to 0.5% for an increase of 1 g/lit. in the concentration of sodium carbonate and by 0.5 to 2% for an increase of 1 g/lit. (as carbon) in the concentration of an organic acid salt.
(2) The crystals of the precipitated alumina hydrate become finer in proportion as the concentration of sodium oxalate present in the Bayer liquor increases. As a result, the efficiency of separation of the precipitated alumina hydrate from the Bayer liquid is proportionally degraded and, accordingly, the amount of the alumina hydrate remaining in the liquid being circulated through the production system increases.
For the purpose of preventing the decrease in grain size of the precipitated alumina hydrate, there have been suggested a method (U.S. Pat. No. 2,935,376) which involves heating the seed added to the liquid and a method which adopts a higher temperature for the precipitation. Both methods inevitably impair the efficiency of the production of alumina.
(3) As the concentration of sodium oxalate increases, the deposition of scale on the inner wall surface of the reaction vessels and pipes grow conspicuous and the period of the continuous operation of the production system is shortened greatly.
(4) As the concentration of carbon compounds increases, the liquid changes color (to a blackish brown, for example) and is opacified. Further there is a possibility of the liquid growing viscous so much as to impede the sedimentary separation of the alumina hydrate. The liquid, consequently, gains in foaming property. Moreover, the capacity for the sedimentary separation of red mud is impaired.
(5) When the alumina hydrate is used as a final product, particularly as a filler for other products, it is desired to possess the highest possible whiteness. The presence of carbon compounds imparts a color to the alumina hydrate to lower the degree of whiteness and impair the commercial value.
The necessity for keeping the total concentration of carbon compounds in the Bayer liquid at a low level derives not only from the need to attain good efficiency of production and quality of product but also from the need to meet increasingly tight pollution controls over plant effluents for the sake of the preservation of the environment. Many of alumina production plants make it a rule to discard the red mud slurry resulting from the last stage of washing not in the sea but on land. The most popular way of disposal consists in dumping the slurry in large artificial ponds and allowing the supernatant occurring in such ponds to be released into natural bodies of water located near the ponds. In this system of disposal, the volume of pollutants such as COD and soda compounds contained in the supernatant released finally into the nearby natural bodies of water is subject to severe control. Thus, the concentration and amounts of such substances must be maintained at a low level. For this reason, the red mud slurry must be subjected to the treatments such as of filtration, concentration and washing before it is dumped in the artificial ponds. The liquid which emanates from these treatments contains alkalis. To prevent the loss of these alkalis, this liquid must be returned to the production system in the plant. This liquid also contains carbon compounds. Thus, the return of this liquid to the system results in further accumulation of carbon compounds in the Bayer liquid.
This problem has some bearing upon the supply and demand situation regarding the ore as a raw material. Recently it has become increasingly more difficult to obtain gibbsitic bauxite which enjoys ready solubility and supply of sparingly soluble boehmitic diasporic bauxite is gradually growing in volume. For effective treatment, the sparingly soluble bauxite must be dissolved at a higher temperature under high pressure. As an inevitable consequence, the ratio of solution of carbon compounds of the ore in the soda liquid increases and the concentration of carbon compounds in the Bayer liquid increases all the more.
When alumina is manufactured by the Bayer Process, therefore, the hindrance caused by carbon compounds is aggravated. For this reason, there has been a strong need for establishment of a process for decisively more effective removal of carbon compounds from the Bayer liquor.
Some methods have been suggested with a view to removing of the carbon compounds and a serious obstacle to the manufacture of alumina by the Bayer Process has been overcome by these methods. Examples are a pressurized oxygen method (Japanese Patent Publication No. 30548/1970), an ultraviolet ray irradiation method (Japanese Patent Laid-open Publication No. 20097/1974), a cooling method (U.S. Pat. No. 3,508,884), a magnesium hydroxide coprecipitation method (Japanese Patent Laid-open Publication No. 130692/1976), a causticizing method (U.S. Pat. No. 3,120,996 and U.S. Pat. No. 3,341,286), a sodium oxalate removing method (U.S. Pat. Nos. 3,649,185 and 3,372,985 and Japanese Patent Publications No. 11480/1973, No. 398/1978 and No. 400/1978) and a bauxite burning method (Japanese Patent Laid-open Publication No. 21395/1972). Of these methods, the causticization method and the sodium oxalate removing method have so far been commercially applied. The principle of the causticizing method comprises converting the sodium carbonate in the Bayer liquor with slaked lime into caustic soda and calcium carbonate and separating calcium carbonate and removing it from the liqiud. The causticizing method may embrace an additional step of evaporating the Bayer liquor, either alone or in combination with another step of further effecting salting out. In any event, the treatment of the liquid in this method must be carried out at a low caustic concentration, and accomplished with poor efficiency in its removal. Moreover, the method is, as a practical matter, capable of removing only sodium carbonate.
The bauxite burning method is based on a procedure comprising the steps of crushing bauxite ore, calcining the crushed ore under an oxidizing atmosphere thereby removing the organic compounds from the ore by virtue of combustion, and thereafter subjecting the remaining ore to digestion in conjunction with caustic soda. Since the ore itself is calcined, this method inevitably calls for use of a large furnace for this calcination and entails consumption of a large volume of fuel. Besides, the calcination degrades the yield of alumina extraction in digestion.
In the case of the sodium oxalate removing method, since sodium oxalate is precipitated in the form of fine particles in conjunction with the alumina hydrate in the stage for the precipitation of alumina hydrate, this method, in the stage for the filtration of the alumina hydrate, separates sodium oxalate in the form of an aqueous solution from the liquid through treatments such as filtration and washing, converts the sodium oxalate with added lime into calcium oxalate and caustic soda, and finally removes the formed calcium oxalate out of the system. The precipitation and separation of sodium oxalate is accomplished by suitably adjusting the concentration of the liquid and the temperature condition. Actually, however, it is difficult to remove the sodium oxalate so as to lower the concentration of the residual sodium oxalate below a certain level. In addition, this method has the disadvantage that it inevitably entails addition of water to the system for the purpose of dissolving sodium oxalate. Japanese Patent Publication No. 92900/1977 and French Pat. No. 75 32169 disclose a barium method which aims to educe and separate the carbonate radical and the oxalate radical in the form of barium salts by addition of barium ion. This method has the disadvantage that it necessitates use of expensive barium and the process itself is rather complicate.
U.S. Pat. No. 4,036,931 and Japanese Patent Laid-open Publication No. 1700/1978 teach a method which comprises causing carbon dioxide gas to react upon the Bayer liquor to which the supernatant formed on the red mud pond has been recycled thereby producing sodium dawsonite, NaAl(OH).sub.2 CO.sub.2, in the form of a precipitate, removing the settled precipitate, decomposing the removed precipitate by calcination into sodium aluminate and recycling sodium aluminate to the Bayer Process. Although this method proves to be effective in the recovery of soda components in the supernatant in the red mud pond, it is totally ineffective in the removal of carbon compounds, particularly organic compounds, from the Bayer liquor.
An object of the present invention is to provide a method which enables desired removal of carbon compounds to be accomplished with higher efficiency than the various methods described above without reference to the kind of carbon compounds.
Another object of the present invention is to establish a method for efficient removal of carbon compounds in the manufacture of alumina by the Bayer Process to thereby heighten the efficiency of the production of alumina by the Bayer Process and prevent possible pollution of the environment with the effluent emanating from the alumina production.
Yet another object of this invention is to provide a method for the manufacture of alumina hydrate of high whiteness and solid and liquid sodium aluminate of high purity.